The invention disclosed and claimed herein generally pertains to improved apparatus for removing heat generated in an X-ray tube, as X-rays are produced thereby. More particularly, the invention pertains to heat removal apparatus of such type which employs a liquid metal having high thermal conductivity. Even more particularly, the invention pertains to apparatus of such type which has comparatively high tolerance to vibration and to misalignment of the anode support shaft.
As is well known in the art, in the operation of an X-ray tube a stream of electrons is directed across very high voltage, such as 100-140 KV, from a cathode to a focal spot position on a tungsten anode target. X-rays are produced as electrons strike the tungsten. However, the conversion efficiencies of such tubes are quite low. Accordingly, considerable heat is generated around the anode, as a byproduct of X-ray production.
In order to reduce heat concentration in regions proximate to the anode, the tungsten target is in the form of an annular track, and the focal spot position comprises a point along the track. The anode is rotatably mounted on a support shaft and rotated at high speeds. By means of such arrangement, the electron stream from the cathode is continually presented with a new and cooler surface. Nevertheless, in a high performance X-ray tube the surface of the anode may reach temperatures in excess of 2500.degree. C., and regions of the anode outside the immediate target surface may rise to temperatures in excess of 1000.degree. C. Thus, in the design of X-ray tubes provision must be made for the continuous removal of substantial amounts of heat. Moreover, the path of heat removal must be directed across a boundary between the anode and the anode bearing shaft, which are rapidly rotating members, and stationary X-ray tube support structure.
In a comparatively recent approach to heat removal, a quantity of liquid metal, referred to as a liquid metal plug, is placed around the anode support shaft at a position lying between two sets of bearings, which journal the shaft and anode for rotation. Such arrangements are generally not intended to bear any of the load which is placed on the shaft by the anode. The plug comprises liquid metal having high thermal conductivity, such as a gallium-tin alloy or the like. The liquid metal plug is intended to provide a heat flow path from the shaft to adjacent stationary support structure. Arrangements of this type are taught, for example, in U.S. patent application Ser. No. 09/134,113, entitled "X-ray Tube Rotor and Stator Assembly", which was filed Aug. 14, 1998 as a continuation in part of U.S. patent application Ser. No. 08/925,294, filed Sep. 8, 1997. Both such applications are commonly assigned herewith to the General Electric Company.
In the past, the problem of leakage has tended to make liquid metal plug designs of the above type unreliable for use in X-ray tubes. Leakage of the liquid metal can contaminate the bearing balls of the bearings which journal the anode shaft, leading to early bearing failure. Leakage can also lead to high voltage breakdown between different components in an X-ray tube, and can cause reduced thermal performance of the liquid metal plug. It will be seen that a liquid metal plug arrangement requires a seal of some type, to retain the liquid metal in contact with the anode support shaft. At the same time, a clearance must be provided between the seal and the shaft, to prevent interference with shaft rotation. However, the tolerance which is allowed for the clearance is very tight. A clearance which is too large will result in leakage, while a clearance which is too small can result in metal to metal contact between stationary and rotating surfaces. Even if the clearance between rotating and stationary components is initially within the allowed tolerance required for successful operation, vibration of the rotating parts is likely to cause misalignment of the anode shaft. Displacement of the shaft could immediately result in the clearance becoming too large or too small. Moreover, the largest displacements induced by vibration tend to occur in regions which are proximate to the anode. As a result, previous liquid metal plug arrangements have generally not been placed in such regions. However, as stated above, the greatest concentration of heat in an X-ray tube is typically located near the anode, and in particular near the anode target. Accordingly, it would by very beneficial to provide a liquid metal plug device which could be positioned in close proximity thereto.